1. Field of the Invention
The present invention relates generally to an encoding/decoding apparatus and method in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to an apparatus and method for transmitting a reverse rate indicator channel (R-RICH) used in a synchronous mobile communication system.
2. Description of the Related Art
In general, a reverse supplemental channel (R-SCH) fundamentally supports a variable rate transmission scheme. In the “variable rate transmission scheme”, a mobile station varies its transmission rate arbitrarily. Generally, a change in a data rate causes a change in a code rate of error correcting codes used in constructing a frame, a symbol repetition frequency, and a length and type of Walsh codes for spreading. Therefore, the mobile station should inform a base station of a data rate of the currently transmitted reverse supplemental channel, so that a base station receiver can correctly receive the reverse supplemental channel. A channel defined for this use is called a reverse rate indicator channel (R-RICH).
The number of kinds of data rates that can be transmitted by the mobile station over the reverse supplemental channel depends upon the number of reverse supplemental channels that can be simultaneously used by the mobile station. The number of the reverse supplemental channels is determined by the base station considering an amount of data to be transmitted over a reverse link during call setup, and then reported to the mobile station. Therefore, the number of information bits transmitted over the reverse rate indictor channel varies depending on the number of the reverse supplemental channels. That is, when the number of the reverse supplemental channels is 1, the mobile station informs a reverse data rate using 4 bits. Further, when the number of the reverse supplemental channels is 2, the mobile station informs the reverse data rate using 7 bits. Since the number of the reverse supplemental channels that can be simultaneously used by the mobile station cannot be changed until a separate command is received from the base station, the mobile station transmits either 4-bit information or 7-bit information over the reverse rate indicator channel. That is, the mobile station will never transmit both the 4-bit information and the 7-bit information at the same time. Conventionally, a (24,4) or (24,7) code is defined as an error correcting code to be used in the reverse rate indicator channel.
A disadvantage of the reverse rate indicator channel (R-RICH) lies in that the number of bits transmitted for transmission of the reverse rates depends upon only the number of the reverse supplemental channels. That is, the number of different data rates that can be transmitted by the mobile station over the reverse supplemental channel is not considered in determining the number of bits transmitted over the reverse supplemental channel. When the number of the transmission bits is determined regardless of the number of different data rates that can be transmitted by the mobile station over the reverse supplemental channel, the mobile station may transmit more bits than actually required bits. For example, when the number of reverse supplemental channels is 1 and the number of different (or “kinds of”) data rates that can be transmitted by the mobile station over the reverse supplemental channel is 4, the minimum number of bits needed for transmitting the data rates is 2. However, in the prior art, when the number of the reverse supplemental channels is 1, the minimum number of the bits required for transmitting the data rates is 4.
In the conventional method in which the number of bits to be transmitted over the reverse rate indicator channel depends upon the number of the reverse supplemental channels, the number of the bits transmitted over the reverse rate indicator channel is larger than needed. Transmitting unnecessarily large number of the bits over the reverse rate indicator channel causes an increase in a code rate of an encoder, making it difficult to apply an optimal encoding method.
FIG. 1 illustrates a structure of an R-RICH transmitter. Referring to FIG. 1, an encoder 100 encodes a 4-bit or 7-bit input rate indicator and outputs 24 coded symbols. A symbol repeater 110 repeats 16 times the 24 coded symbols provided from the encoder 100. A signal mapper 120 performs signal mapping on the coded symbols output from the symbol repeater 110 by mapping 0 to 1 and 1 to −1. A spreader 130 spreads the signal-mapped symbols.
As illustrated in FIG. 1, the rate indicator has 4 bits or 7 bits, and encoded into 24 coded symbols before transmission. When errors occur during transmission of the rate indicator encoded into the coded symbols, the rate indicator may incorrectly indicate the code rate of the corresponding reverse supplemental channel, the symbol repetition frequency, and the length and type of the spread Walsh code. As a result, the receiver cannot correctly analyze the reverse supplemental channel. Therefore, the rate indicator should be encoded by a (24,4) or (24,7) encoder having good performance. In addition, the rate indicator should be decoded as fast as possible in order to analyze the corresponding supplemental channel.